Rotary screw compressors are used to compress a wide variety of gases. These compressors are used for industrial refrigeration and for air conditioning. They are also used for air compression and for compressing gases such as natural gas. In these circumstances, the screw compressors may be located in remote locations and required to run for long periods of time, weeks or months, with minimal service. These compressors are also used to compress a wide variety of gases for other industrial applications.
Screw compressor may utilize two rotors on parallel axes, referred to as twin screw designs or may be single screw compressors or single screw designs. Twin screw designs employ two matched helical rotors, a male rotor and a female rotor, which turn together. A single screw design employs a single rotating helical rotor that usually meshes with gate rotors. In oil injected screw compressor types, the screw compressors operate with oil injection that seals the clearance between the rotors and between the rotors and the cylinder. The lubricant also helps cool the compressor. The lubricant, which is in contact with the refrigerant or gas, may absorb the refrigerant or gas when the lubricant used is miscible with the refrigerant or gas being compressed, which is frequently the case.
Large screw compressors have experienced problems with excessive vibration, particularly when driven at high drive power, in excess of 3000 horsepower, and high discharge pressures. This problem may be brought about as a result of rotor excitation near critical speeds. Forces imposed on the rotor are resisted by the use of radial and axial (thrust) bearings that position the rotors in the supporting housings. The existing bearing designs may not provide sufficient damping to avoid rotor excitation. These radial and axial bearings generally comprise two basic types: hydrodynamic bearings and anti-friction bearings. Hydrodynamic bearings, or oil film type bearings include sleeve or journal bearings. These bearings do not provide precise rotor positioning and result in higher frictional power consumption, which results in higher temperatures that can limit operations. These factors are exacerbated as the compressor size increases, limiting their usefulness in larger compressors. These bearings also require full time oil pumps to supply lubricant to the bearings. Anti-friction bearings such as ball bearings, cylindrical roller-type bearings or tapered roller bearings overcome these problems. They desirably provide precise rotor positioning. However, anti-friction bearings disadvantageously provide much lower relative damping coefficients than hydrodynamic bearings. In situations where vibratory motion of the rotors with respect to the housing occurs, anti-friction bearings provide very little relative damping of the motion. Damping is a force produced in the supporting or damping element that resists the relative motion of the rotors. The magnitude of the resisting force is proportional to the velocity of motion. Thus, as sizes of the screw compressors increase, and damping forces increase, the anti-friction bearings become less effective in damping vibrations. These vibrations may be amplified by resonance at critical frequencies that are excited by energy at a lobe-passing frequency or one of its harmonics. This can result in damage to not only the male and female rotors, but also to the rotor housing as the rotor bounces into contact with the housing as well as applying load to thrust bearings, all of which may result in damage to the compressor.
Squeeze film dampers are dampening devices that have been used to provide viscous damping in mechanical systems. These squeeze film dampers have been used in series with conventional bearings set forth above in rotating machinery such as jet engines. These squeeze film dampers provide structural isolation to reduce, for example, jet engine rotor response to imbalance. However, squeeze film dampers such as utilized in jet engines have not been utilized in screw compressors as the squeeze film dampers positioned in series with conventional bearings do not provide the needed accuracy for positioning the rotor in a screw compressor. The clearance in such systems is too great to accurately support screw rotors during operation.
What is needed is a damping mechanism for screw compressors that dampen rotor vibration so that resonance effects are limited at critical frequencies, while also providing precise rotor positioning within the housing and with respect to mating rotors.